<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <style>
        #demo{
            border: 10px solid #000;
            border-bottom-color: red;
            width: 0px;
            height: 0px;
        }
        .arrow{
            position: relative;
            height: 30px;
            width: 100px;
            background-color: #ccc;
            margin-top: 10px;
        }
        .arrow:after,.arrow:before{
            border: 10px solid transparent;
            width: 0px;
            height: 0px;
            content:' ';
            position: absolute;
            left: 40px;
        }
        #demo1:after{
            border-bottom-color:  #f00;
            top:-19px;
            left: 40px;
            
        }
        #demo1:before {
            border-bottom-color: #00f;
            top: -20px;
            left: 40px;
        }
        
        #demo2:before {
            border-bottom-color: #ccc;
            top: -20px;
        }
        #demo2:after{
            border-bottom-color:  transparent;
            top:-19px;
        }
    </style>
</head>
<body>
    <div id="demo"></div>
    <div id="demo1" class="arrow"></div>
    <div id="demo2"  class="arrow"></div>
</body>
</html>